Despawn entities on relationship component removal when linked spawn is enabled

## What problem does this solve or what need does it fill?

The following code works:

```rust
let mut world = World::new();

let mut entity = world.spawn((
Name::new("Root"),
children![Name::new("Child1"), Name::new("Child2"),],
));

entity.remove::();

assert_eq!(world.entities().len(), 3);
```

But I'm not sure if this behavior is expected/desired. This makes it easy for users to "leak" entities.

## What solution would you like?

We have [RelationshipTarget::LINKED_SPAWN](https://docs.rs/bevy/latest/bevy/prelude/trait.RelationshipTarget.html#associatedconstant.LINKED_SPAWN) which works for despawn and cloning. I'd suggest to extend it to the relationship target component removal.